C# Types and variables (C# Specification Part 2)

There are two kinds of types in C#: value types and reference types. Variables of value types directly contain their data whereas variables of reference types store references to their data, the latter being known as objects. With reference types, it is possible for two variables to reference the same object and thus possible for operations on one variable to affect the object referenced by the other variable. With value types, the variables each have their own copy of the data, and it is not possible for operations on one to affect the other (except in the case of ref and out parameter variables).

C#’s value types are further divided into simple types, enum types, struct types, and nullable types, and C#’s reference types are further divided into class types, interface types, array types, and delegate types.

The following table provides an overview of C#’s type system.

Category		Description
Value types	Simple types	Signed integral: sbyte, short, int, long
		Unsigned integral: byte, ushort, uint, ulong
		Unicode characters: char
		IEEE floating point: float, double
		High-precision decimal: decimal
		Boolean: bool
	Enum types	User-defined types of the form enum E {...}
	Struct types	User-defined types of the form struct S {...}
	Nullable types	Extensions of all other value types with a null value
Reference types	Class types	Ultimate base class of all other types: object
		Unicode strings: string
		User-defined types of the form class C {...}
	Interface types	User-defined types of the form interface I {...}
	Array types	Single- and multi-dimensional, for example, int[] and int[,]
	Delegate types	User-defined types of the form e.g. delegate int D(...)

The eight integral types provide support for 8-bit, 16-bit, 32-bit, and 64-bit values in signed or unsigned form.

The two floating point types, float and double, are represented using the 32-bit single-precision and 64-bit double-precision IEEE 754 formats.

The decimal type is a 128-bit data type suitable for financial and monetary calculations.

C#’s bool type is used to represent boolean values—values that are either true or false.

Character and string processing in C# uses Unicode encoding. The char type represents a UTF-16 code unit, and the string type represents a sequence of UTF-16 code units.

The following table summarizes C#’s numeric types.

Category	Bits	Type	Range/Precision
Signed integral	8	sbyte	–128...127
	16	short	–32,768...32,767
	32	int	–2,147,483,648...2,147,483,647
	64	long	–9,223,372,036,854,775,808...9,223,372,036,854,775,807
Unsigned integral	8	byte	0...255
	16	ushort	0...65,535
	32	uint	0...4,294,967,295
	64	ulong	0...18,446,744,073,709,551,615
Floating point	32	float	1.5 × 10−45 to 3.4 × 1038, 7-digit precision
	64	double	5.0 × 10−324 to 1.7 × 10308, 15-digit precision
Decimal	128	decimal	1.0 × 10−28 to 7.9 × 1028, 28-digit precision

C# programs use type declarations to create new types. A type declaration specifies the name and the members of the new type. Five of C#’s categories of types are user-definable: class types, struct types, interface types, enum types, and delegate types.

A class type defines a data structure that contains data members (fields) and function members (methods, properties, and others). Class types support single inheritance and polymorphism, mechanisms whereby derived classes can extend and specialize base classes.

A struct type is similar to a class type in that it represents a structure with data members and function members. However, unlike classes, structs are value types and do not require heap allocation. Struct types do not support user-specified inheritance, and all struct types implicitly inherit from type object.

An interface type defines a contract as a named set of public function members. A class or struct that implements an interface must provide implementations of the interface’s function members. An interface may inherit from multiple base interfaces, and a class or struct may implement multiple interfaces.

A delegate type represents references to methods with a particular parameter list and return type. Delegates make it possible to treat methods as entities that can be assigned to variables and passed as parameters. Delegates are similar to the concept of function pointers found in some other languages, but unlike function pointers, delegates are object-oriented and type-safe.

Class, struct, interface and delegate types all support generics, whereby they can be parameterized with other types.

An enum type is a distinct type with named constants. Every enum type has an underlying type, which must be one of the eight integral types. The set of values of an enum type is the same as the set of values of the underlying type.

C# supports single- and multi-dimensional arrays of any type. Unlike the types listed above, array types do not have to be declared before they can be used. Instead, array types are constructed by following a type name with square brackets. For example, int[] is a single-dimensional array of int, int[,] is a two-dimensional array of int, and int[][] is a single-dimensional array of single-dimensional arrays of int.

Nullable types also do not have to be declared before they can be used. For each non-nullable value type T there is a corresponding nullable type T?, which can hold an additional value null. For instance, int? is a type that can hold any 32 bit integer or the value null.

C#’s type system is unified such that a value of any type can be treated as an object. Every type in C# directly or indirectly derives from the object class type, and object is the ultimate base class of all types. Values of reference types are treated as objects simply by viewing the values as type object. Values of value types are treated as objects by performing boxing and unboxing operations. In the following example, an int value is converted to object and back again to int.

using System;

class Test
{
      static void Main() {
            int i = 123;
            object o = i;                 // Boxing
            int j = (int)o;         // Unboxing
      }
}

When a value of a value type is converted to type object, an object instance, also called a “box,” is allocated to hold the value, and the value is copied into that box. Conversely, when an object reference is cast to a value type, a check is made that the referenced object is a box of the correct value type, and, if the check succeeds, the value in the box is copied out.

C#’s unified type system effectively means that value types can become objects “on demand.” Because of the unification, general-purpose libraries that use type object can be used with both reference types and value types.

There are several kinds of variables in C#, including fields, array elements, local variables, and parameters. Variables represent storage locations, and every variable has a type that determines what values can be stored in the variable, as shown by the following table.

Type of Variable	Possible Contents
Non-nullable value type	A value of that exact type
Nullable value type	A null value or a value of that exact type
object	A null reference, a reference to an object of any reference type, or a reference to a boxed value of any value type
Class type	A null reference, a reference to an instance of that class type, or a reference to an instance of a class derived from that class type
Interface type	A null reference, a reference to an instance of a class type that implements that interface type, or a reference to a boxed value of a value type that implements that interface type
Array type	A null reference, a reference to an instance of that array type, or a reference to an instance of a compatible array type
Delegate type	A null reference or a reference to an instance of that delegate type